1. Field of the Invention
This invention relates to a vibration-isolating bushing, for example, for use in an automobile, in part of a suspension mechanism of which it is incorporated to alleviate vibrations.
2. Description of the Related Art
In automotive vehicles, with a view to controlling vibrations transmitted from the wheel side to the automotive body side or vibrations transmitted from the engine side to the body side, a vibration-isolating bushing is generally incorporated in part of a suspension mechanism or an engine support mechanism. One example of such vibration-isolating bushings is a so-called bulge type of vibration-isolating bushing, which is provided with a bulge portion bulging out in an axially square direction in an axially central area of an inner cylinder so as to lower the spring constant in a prying direction while heightening the spring constant in the axially square direction.
FIG. 6 illustrates an example of a conventional bulge bushing. This vibration-isolating bushing is composed of an inner cylinder 102 made of metal provided with a bulge portion 101 bulging out in an axially square direction in an axially central part thereof, an outer cylinder 103 encircling the inner cylinder, and a rubber-like elastomer 104 interposed between both cylinders 102, 103. Due to the bulge portion 101, the rubber-like elastomer 104 is made thinner in its axially central part than in its both ends, whereby the spring constant in the axially square direction A (an orthogonal direction to the axial direction) is set to be larger and the spring constant in a prying direction B (a direction in which center axes of the inner and outer cylinders 102, 103 slant relatively) is set to be smaller. Thereby a compromise or trade-off between an improvement in riding comfort and an enhancement in steering stability is contemplated.
In the known vibration-insulating bushing shown in FIG. 6, the bulge portion 101 of the inner cylinder 101 is integrally formed by forging. This renders the processing cost high and causes the product weight to increase. Further when forging, it is difficult to make the outside diameter of the bulge portion large beyond a definite size by forming and consequently, the degree of freedom in design between the spring constant in the axially square direction and the spring constant in the prying direction is low.
It is therefore proposed to fashion the bulge portion by an annular cover made of a synthetic resin. Such a vibration-isolating bushing is proposed in JP Utility Model Laid-Open Publication 5-64544 (1993) as shown in FIG. 7, wherein the annular cover 112 made of a synthetic resin is provided rigidly as a bulge portion on the outer periphery of an inner cylinder 110 at its central area, and a rubber-like elastomer is interposed between the inner cylinder 110 and an outer cylinder 114 so as to sheathe the annular cover 112. When the annular cover 112 is provided on the outer periphery of the inner cylinder 110 in this way, at the time of molding the rubber-like elastomer 116 by vulcanization, the annular cover 112 can be displaced owing to its heat and pressure. Consequently, the aforementioned Publication discloses that as a stop for disconnection or detachment of the annular cover 112, two to four recesses 118 are circumferentially defined on the outer periphery of the inner cylinder 110 and filled with a part of the synthetic resin or the disconnection stop is inversely provided in a projecting manner, thereby securing the annular cover 112.
Again, JP Utility Model Laid-Open Publication 6-76729 (1994) discloses that in order to prevent the displacement of the annular cover 112 in the axial direction or a rotational direction, a plurality of seating faces 120 each having a length close to the breadth of the annular cover 112 and a flat bottom are circumferentally recessed, as illustrated in FIG. 8, on the outer periphery of the axially central part of the inner cylinder 110 and upon molding of the annular cover 112, a synthetic resin is flowed into the seating faces 120 thereby to seat and secure the annular cover 112 on the seating faces 120.
With these vibration-isolating bushings as illustrated in FIGS. 7 and 8, a plurality of recesses, which are to some degree large, are thus provided on the inner cylinder for the prevention of displacement of the annular cover, as a result of which the buckling strength of the inner cylinder in the axial direction to loads comes to decline. Further the conventional displacement preventive structures as exemplified in FIGS. 7 and 8 are low-cost as compared to the case of forging in FIG. 6, yet necessitate more or less several steps for molding and a further cost cutting is therefore required. Moreover, cutting wastage is produced by the provision of the recess portions and an improvement from an environmental aspect is also required, accordingly.